Electromechanical reed system



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@if @JW United States Patent i" 3,024,429 ELECTROMECHANCAL REED SYSTEMAlbert L. Cavalieri, Jr., Hatboro, Pa., and Robert W.

Roop, Sewell, NJ., assignors, by mesne assignments, to PhilcoCorporation, Philadelphia, Pa., a corporation of Delaware Originalapplication May 29, 1953, Ser. No. 358,286, now Patent No. 2,875,353,dated Feb. 24, 1959. Divided and this application Jan. 14, 1959, Ser.No. 786,832

4 Claims. (Cl. S33- 72) This application is a division of our copendingapplication Serial No. 358,286, led May 29, 1953, now Patent No.2,875,353.

This invention relates to frequency sensitive electromechanical systems,and more particularly to electromechanic-al systems employing avibratory reed as the frequency sensitive element.

It is well known that a reed comprising an elongated strip of elasticmaterial rigidly supported at one end may be caused to vibrate by theapplication thereto of a periodic excitation having a frequency in thevicinity of the natural resonant frequency of the reed. The amplitude ofthe vibration, that is the maximum displacement of the free end of thereed measured from the rest position, is a function of many variablesincluding the constants of the reed, the amplitude of the excitingsignal and the difference between the frequency of the exciting signaland the natural frequency of the reed.

One relationship between the amplitude of vibration of the reed and thefrequency of the exciting signal is known as the bandwidth of a reed.The bandwidth of a reed is dened as the frequency interval between theexciting frequency below the resonant frequency of the reed and theexciting frequency above the resonant frequency of the reed, both ofwhich cause an amplitude of vibration equal to .707 times the amplitudeof vibration caused by an exciting signal of the same amplitude having afrequency equal to the resonant frequency of the reed. It will berecognized that the term bandwidth, as applied to a reed, has a meaningclosely analogous to that of the term bandwidth as applied to a resonantresistor-inductor-capacitor circuit. It is also known that, by properchoice of material and dimensions, reeds may be constructed which have abandwidth that is very small compared to the resonant frequency. Thequality factor or Q is the ratio of the energy stored in the vibratingreed system to the energy dissipated per cycle, and is very nearly equalto the ratio of the resonant frequency to the bandwidth for a reedhaving a high Q. By way of example, it is an easy matter to construct avibrating reed having a resonant frequency of the order of 250 cyclesper second and a bandwidth of a small fraction of one cycle per second.

Frequency meters are now commercially available which employ a bank ofhigh Q reeds, each tuned to a slightly different resonant frequency. Thereeds are energized by forming them of a magnetic material and applyingthe periodic signal to be measured to an electromagnet spaced somedistance from the reed bank. The frequency is determined by observingthe reeds and noting visually which reed is vibrating with the greatestamplitude. This and other similar vibratory reed systems known to theprior art are subject to several serious disadvantages. The magneticdrive systems, `and other drive systems known in the art, areinefficient and difficult to control and are seriously affected bychanges in operating temperature, humidity, etc. A second disadvantageis that the output of most prior art reed systems is in the form of avisual indication which cannot be utilized to control an electricalcircuit. In certain prior art reed systems, contact points are providedwhich contact the 3,024,429 Patented Mar. 6, 1962 reed if the amplitudeof vibration thereof exceeds a preselected value. Such contact pointsare ditlicult to adjust, they provide only a limited amount of outputinformation, and they interfere with the natural vibration of the reed.Attempts have been made to obtain a measurement of the amplitude ofvibration by measuring the variation in the capacitance between the freeend of the reed and the stationary plate. Such a system is utterlyimpractical if several reeds of I small size are to be mounted within asmall volume. These and other disadvantages have limited the usefulnessof the vibrating reed type of electromechanical systems.

Therefore it is an object of the present invention to provide a new andimproved type of vibrating reed structure.

It is a further object of the invention to provide an improved reedstructure in which the input and/or the output are in the form ofelectrical signals.

It is -a further object of the invention to provide a novelelectromechanical ter network having an input and/or output in the formof an electrical signal.

These and other objects of the invention are accomplished through theuse of a vibratory reed structure comprising one or more elongatedvibratory reed elements each having one end thereof secured to a base.The other end of the reed element is free to vibrate at the naturalresonant frequency of the reed element. One or more piezoelectricmembers are secured to each reed element in a region adjacent the fixedend thereof so as to be strained by the vibrations of the reed element.The energizing signal is supplied to, or the output signal is obtainedfrom, electrodes suitably positioned on the piezoelectric material. Incertain preferred embodiments of the invention `a plurality ofpiezoelectric members are employed which are mechanically coupled to oneanother through the vibratory reed element. The reed elements may beemployed either singly or in matched pairs with mechanical couplingtherebetween, and the individual reed elements or reed element pairs maybe assembled in banks.

For a better understanding of the invention together with other andfurther objects thereof reference should now be made to the followingdetailed description which is to be read in connection with theaccompanying drawing in which:

FIG. l is an isometric view of one embodiment of the present invention;

FIG. 2 is an isometric view of a second embodiment of the inventionemploying two reed elements in a balanced system;

FIG. 3 is an assembly of three reed systems of the type shown in FIG. 2;and

FIGS. iA-4D are plots showing a few of the many characteristicsobtainable with the assembly shown in FIG. 3.

The embodiment of the invention shown in FIG. l comprises a reed element`40 which is formed integrally with or is rigidly fastened to a base 42.Reed element 40 may be formed of any material having a relatively highmodulus of elasticity and relatively low internal losses. The length,thickness, and stiffness of the material should be selected inaccordance with conventional engineering practice to give the desiredresonant frequency. In the embodiment shown in FIG. 1, reed element `40may be formed of a magnetic or nonmagnetic material. lt is sometimesadvantageous, for reasons that will appear presently, to form reedelement `40 of a non-conducting material such as glass. Two members 44and 46, formed of piezoelectric material, are bonded to reed element 40adjacent the fixed end thereof. Barium titanate and Rochelle salt aretwo materials known to exhibit a suitable piezoelectric eect Bariumtitanate is suggested as a preferred choice of material in mostinstances. Barium titanate is made to exhibit the piezoelectric effectby first forming the material in the desired shape, for example in theshape of members 44 and 46, applying suitable electrodes to two faces ofthe block, polarizing the barium titanate by applying a D C. voltage tothe two electrodes while the block is held at a temperature above theCurie point of the material, and then allowing the material to coolbelow the Curie point before removal of the polarizing potential.

The bonding of members 44 and 46 to reed element 40 may be accomplishedby the use of a thermosetting bonding material which is characterized byhaving low shrinkage during curing. Araldite Powder, manufactured by theCiba Company, is a suitable example. In constructing the embodimentshown in FIG. 1 it may be desirable to polarize members 44 and 46 afterassembly since the heat necessary to tire the electrodes and to bondmembers 44 and 46 and element 4t) might cause depolarization of members44 and 46. Members 44 and 46 are provided with conductive areaelectrodes, such as fired platinum electrodes, on the exposed verticalfaces 48` and 50. The faces of members 44 and 46 which are in contactwith reed element 40 are also provided with conductive area electrodes.If reed element 46 is formed of a conductive material, and members 44and 46 are in electrical contact therewith (which will be the case if aconductive bonding agent is employed), then reed element 40 will serveas the second area electrode for members 44 and 46. The advantage ofemploying a nonconductive reed or a nonconducting bonding agent is that,in this form of construction, members 44 and 46, which form portions ofthe input and output circuits, respectively, are electrically isolated.Leads 52 are connected to the two conductive electrodes on member 44,and leads S4 are connected to the conductive electrodes on member 46.Members 44 and 46 are polarized in the direction of a line joining thetwo faces but the polarization of the two members 44 and 46 may be inthe same or opposite directions. The neutral axis of the reed structureshown in FIG. l lies somewhere within reed element 40 if members 44 and`46 are of approximately equal thickness. It is desirable to form thesetwo members with substantially the same dimensions in order to preservethe symmetry of the vibrating1 system. Members 44 and 46 are shown ashaving a rectangular form, but the invention is not to be limited tothis particular configuration. In some instance it may be desirable totaper members 44 and 46 and/ or reed element 40 to provide a moreuniform straining of members 44 and 46. Alternatively, it would bepossible to form reed element 40 with an elliptical cross-section andmembers 44 and 46 with a crescent shaped cross-section. Many otherconfigurations will occur to those skilled in the art of designing reedsystems.

The operation of the system shown in FIG. 1 will now be explained. Anenergizing signal is supplied to leads 52. This signal will stressmember 44 but no substantial movement of reed element 40 will resultunless the energizing signal has a component at a frequency which issubstantially equal to the resonant frequency of the lreed system. Ifreed element 4G is not set into vibratlon, no signal appears betweenoutput leads 54. However, if the energizing signal supplied to leads S2has a component at a frequency equal to the resonant frequency of reedelement 40, the periodic stresses applied to reed element 40 by member44 will cause reed element 40 to vibrate. If reed element 4l) is causedto vibrate, these vibrations will strain member 46 and an output signalwill appear between leads S4. The amplitude of this output signal willbe a function of the amplitude of the vibration of reed element 46 whichis, in turn, a function of the amplitude of the input signal, and thedifference in frequency between the frequency of the component causingexcitation and the resonant frequency of the reed system. It should beremembered that, in any instance where an applied potential causes astrain in a piezoelectric member, a corresponding strain resulting froman externally applied stress will cause a potential to appear at thepoints where the potential was formerly applied. Therefore, throughoutthe specification and claims, any piezoelectric member described as ameans for deriving a signal from a reed system may also be employed tosupply energy to the reed system.

FIG. 2 illustrates a preferred form of the invention which employs tworeed elements 6i) and 62 arranged in a balanced system. Reeds 60 and 62preferably have substantially the same dimensions and identical resonantfrequencies. Piezoelectric members 66 and 68 are secured to oppositesides of reed element 60 in the plane of motion of this reed, andpiezoelectric members 70 and 72 are secured to opposite sides of reedelement 62. Reed elements 60 and 62 are mechanically coupled by a block74, of material having a high modulus of elasticity, which is disposedtherebetween and bonded to piezoelectric members 68 and 70. The wholeassembly just described is secured ito base 76 in any suitable manner.The mechanical coupling afforded by block 74 causes reed elements 6) and62 to vibrate 180 out of phase like the arms of a tuning fork. Members68 and '70 have their exposed faces polarized alike s0 that like chargesappear on these faces. Therefore members 68 and 70 may be connected inparallel as shown in FIG. 2. Similarly members 66 and 72 may beconnected in parallel as shown. Various modifications of the systemshown are possible. The polarization of members 66 and 68 may bereversed and members 66 and 70 and 68 and 72 connected in parallel.Obviously, members 63 and 70 shown in FIG. 2 may be connected to theinput terminals of the system and thus serve as the driving members ofthe system, in which case members 66 and 72 would be connected to theoutput terminals. In the embodiment shown in FIG. 2, members 66 and 72may differ in size or shape from members 68 and 70 without destroyingthe symmetry of the system. This makes it possible te design one pair ofpiezoelectric members to give maximum efiiciency as driving elements,and the other pair to give maximum efiiciency as output couplingelements. In an alternative form of the invention, block 74 may besecured directly to, or be formed integrally with, reed elements 69 and62. In this form of the invention, members 68 and 70 may be secured toreed elements 60 and 62 in a position above block 74. Since reedelements 60 and 62 are mechanically coupled through block 74, one pairof piezoelectric members, for example members 66 and 68, may be omitted.However, this destroys the symmetry of the system and removes many ofthe advantages of the balanced system. The chief advantages of thebalanced system are lower impedance because of the parallel connectionof the piezoelectric members, less coupling to base 76 because of thephase difference in the vibrations of reed elements 60 and 62, andreduced shock excitation of reed elements 60 and 62 due to shocksimparted to base 76 from external sources. The reduced shock excitationis due to the fact that any jarring of base 76 excites reed elements 60and 62 in phase causing equal and opposite signals to appear on theexposed faces of members 68 and 70. The parallel connection of blocks 68and 70 causes the signal appearing on one block, due to shockexcitation, to cancel the corresponding signal on the other block. Thelower coupling to base 76 permits a less massive base to be employed andallows systems of the `type shown in FIG. 2 to be assembled into banksof closely spaced systems without appreciable mutual couplingtherebetween.

FIG. 3 shows an electromechanical system employing three pairs of reedelements 60-62, 60-62 and 60-62". Each of these pairs may be tuned tothe same or to slightly different resonant frequencies. Members 66, 66'and 66" are shown connected in parallel as are the other piezoelectricmembers corresponding to members 68, 70 and 72 in FIG. 2. The systemshown in FIG. 3 will not be described in detail since the operationthereof is believed to be obvious from the foregoing detaileddescription of the present invention. One point should be noted,however-namely, ythat it is not necessary that the output piezoelectricelements be polarized alike or connected in parallel in the manner shownunless it is desired to construct a simple filter having a single fiatpassband. Furthermore, the reed element pairs in different systems inthe bank may have different bandwidths to facilitate shaping of thecomposite passband.

FIGS. 4A through 4D show the passbands which may be obtained by suitablecombination of the novel reed systems described above. It should beremembered that the systems having any one of these passbands may haveboth an electrical input and an electrical output, that the systems arerelatively easy and inexpensive to con struct and occupy only a very fewcubic inches of space even at audio frequencies. Obviously it isimpossible to duplicate these results with conventionalcapacitorinductor networks or known forms of reed systems.

So far no mention has been made of the limits of frequency operation ofthe present invention. Again it is both difficult and undesirable to setprecise limits since the development of new materials for the reedelements or the piezoelectric members might extend the range ofoperation beyond present limits. However, it can be safely stated thatthe reed system shown may be constructed of presently availablematerials to cover the range from subaudible frequencies to frequenciesof the order of several kilocycles per second.

While there have been described what are at present considered to be thepreferred embodiments 0f the invention and the manner in which the sameare to be used, it is recognized that other and further modifications ofthe invention are possible which fall within the spirit and scope of thehereinafter appended claims.

What is claimed is:

l. A frequency sensitive electromechanical system comprising first andsecond substantially identical elongated reed elements, each of saidreed elements having a cross-section having a longer and a shortertransverse axis, a base member, each of said first and second reedelements being individually and rigidly secured at a first extreme endthereof directly to said base member, said reed elements being disposedin spaced juxtaposition with the longitudinal axes and the shortertransverse axes thereof in a common plane, rst and second substan-.tially identical piezoelectric members secured to each of said reedelements in positions adjacent said first ends, said first and secondpiezoelectric members. being positioned substantially at opposite endsof said shorter transverse axis of the reed element with which they areassociated, said piezoelectric members being formed with electrodes'thereon to which an electrical signal may be supplied to and derivedfrom piezoelectric members, and means mechanically coupling said firstreed element to said second reed element, said mechanical coupling meanscomprising a block of material having a high modulus of elasticityfitted between and mechanically secured to the adjacent faces of the twocentrally disposed ones of the four abovementioned piezoelectricmembers.

2. A frequency sensitive electromechanical system comprising a basemember, rst and second substantially identical reed elements, each ofsaid reed elements being individually and rigidly secured at a firstextreme end thereof directly to said base member, said reed elementsbeing disposed in spaced juxtaposition and arranged to vibrate in acommon plane, each of said reed elements having first and secondpiezoelectric members secured to the sides thereof at positions adjacentsaid first end thereof, said two piezoelectric members associated wi-theach reed element being positioned so as to be oppositely strained byvibrations of said reed element, said piezoelectric members being formedwith electrodes thereon in regions which undergo changes in potential inrespouse to preselected straining thereof, said first and second reedelements being so disposed that a face of one piezoelectric memberassociated with the first reed element is adjacent face of apiezoelectric member associated with said second reed element, and meansmechanically coupling said first reed element to said second reedelement, said mechanical coupling means comprising a block of materialhaving a high modulus of elasticity fitted between and mechanicallysecured to said two adjacent faces of said piezoelectric members.

3. A frequency sensitive electromechanical system as in claim 2, saidsystem further comprising means electrically connecting one of saidpiezoelectric members associated with said first reed element in shuntwith one of said piezoelectric members associated with said second reedelement, and means electrically connecting the other of saidpiezoelectric members associated with said first reed element in shuntwith the other of said piezoelectric members associated with said secondreed element.

4. An electrical filter circuit comprising a plurality of frequencysensitive electromechanical systems, each system comprising a basemember, `first and second substantially identical reed elements, each ofsaid reed elements -being individually and rigidly secured at a firstextreme end thereof directly to said base member, each of said reedelements having first and second piezoelectric members secured -to thesides thereof at positions adjacent said first end thereof, said twopiezoelectric members associated with each reed element being positionedso as to be oppositely strained 4by vibrations of said reed element,said piezoelectric members being formed with electrodes thereon inregions which undergo changes in potential in response to preselectedstraining thereof, said reed elements being disposed in spacedjuxtaposition and arranged to vibrate in a common plane, said reedelements being further disposed so that a face of one of thepiezoelectric members associated with said first reed element isadjacent face of one of said piezoelectric members associated with saidsecond reed element, means mechanically coupling said first reed elementto said second reed element, said mechanical coupling means comprising ablock of material having a high modulus of elasticity fitted between andmechanically secured to said adjacent faces of said piezoelectricmembers, means connecting each electrode of each frequency sensitiveelectromechanical system to the corresponding electrode in each otherfrequency sensitive electromechanical system, thereby to connectcorrespond ing piezoelectric members in various frequency sensitiveelectromechanical systems in shunt with one another, the reed elementsin different electromechanical systems being resonant at differentfrequencies.

References Cited in the file of this patent UNITED STATES PATENTS1,693,806 Cady Dec. 4, 1928 2,081,405 Mason May 25, 1937 2,185,966Pfanstiehl Jan. 2, 1940 2,666,196 Kinsley et al. Jan. 12, 1954 2,769,867Crownover et al. Nov. 6, 1956

